Enterovirus 71 (EV71) causes hand-foot-mouth disease (HFMD) in children. Unlike related viruses that cause only mild HFMD, EV71 also causes severe neurologic disease, and death- most often due to pulmonary edema and cardiorespiratory failure. Recent epidemics in Asia have affected millions of children, putting a heavy burden on the healthcare system, and causing thousands of deaths. Why some children suffer severe neurologic disease or pulmonary edema is not known, but pulmonary edema and fatal outcome are associated with high levels of inflammatory cytokines. We believe that one critical factor may be the tropism of specific EV71 isolates for P-selectin glycoprotein ligand-1 (PSGL-1), a signaling receptor almost exclusively expressed on blood cells. PSGL-1 mediates the tethering of leukocytes to selectins on vessel walls (6), and PSGL-1 engage- ment initiates a series of intracellular events that promote leukocyte adhesion adhesion and migration into tissues, and lead to the production of cytokines and chemokines by immune cells. The hypothesis underlying this application is that virus interaction with PSGL-1 on leukocytes is important in the pathogenesis of severe EV71 disease, both because it permits virus to enter and infect specific leukocyte populations, and because it triggers PSGL-1-mediated signals that contribute to leukocyte activation, adhesion/migration, and cytokine production. We will test this hypothesis in three sets of experiments. First, we will identify leukocyte subsets that are bound or infected by EV71 in a PSGL-1-dependent manner, and determine whether virus binding or infection leads to immune cell activation, adhesion, or cytokine production. Second, we will determine whether EV71 interaction with leukocytes stimulates the passage of virus and inflammatory cells across an in vitro model of the blood-brain barrier. Finally, in collaboration with investigators at the Pasteur institute of Shanghai, we well determine whether PSGL-1-binding viruses have increased pathogenic potential, by examining virus isolates obtained from children with severe and mild EV71 disease.